Pastrnak leads the Bruins with six points (two goals, four assists) this postseason, and Charlie McAvoy has five assists. Patrice Bergeron and Brad Marchand each scored three goals. Tuukka Rask is 4-1 with a 1.81 goals-against average and .941 save percentage.
Anthony Beauvillier (three goals, four assists) and Jean-Gabriel Pageau (one goal, six assists) lead the Islanders with seven points each this postseason. Brock Nelson and Josh Bailey have scored six points each on three goals and three assists. Rookie goalie Ilya Sorokin got the four wins with a 1.95 GAA and .945 save percentage. Semyon Varlamov lost his two starts with a 3.61 GAA and .903 save percentage.
"We're enjoying this win right now," New York forward Travis Zajac said. "I'm sure in the next day or two we'll get ready to go up against Boston. Obviously, they're playing some good hockey. They beat a really good team, we beat a really good team. It's going to be another fun series to be a part of."
The Bruins were 3-3-2 against the Islanders this season. Marchand (four goals, three assists) and Pastrnak (two goals, five assists) led Boston with seven points each. Rask was 2-2-0 with a 1.86 GAA and .925 save percentage, Jeremy Swayman made 25 saves in a 3-0 win April 16, and Jaroslav Halak was 0-1-2 with a 4.36 GAA and an .859 save percentage.
"They're a patient team. They defend well," Cassidy told NBC Sports Boston on Thursday. "Obviously, we saw it in the first series, two good goaltenders. They had to go to their plan B, and [Ilya Sorokin] won them all four games. It doesn't matter who they put in there, they're going to get good goaltending.
"I think they have a lot of our attributes. They want to be structured, they want to play with discipline, they have good goaltending. Their [defensemen] certainly get involved, but they want to play defense first. They've got different lines that can hurt you. So in that regard, we're playing ourselves a little bit. Very well-coached. So for us, it's a matter of staying within ourselves, play the game in front of you, put some pressure on their D."
Pageau led New York with eight points (five goals, three assists) against Boston, and Mathew Barzal scored seven points (three goals, four assists). Varlamov was 5-1-0 with a 1.93 GAA and .943 save percentage, and Sorokin was 0-1-1 with a 2.23 GAA and .921 save percentage.
It is the third playoff series between the Bruins and Islanders. New York won the first two, the last being the 1983 Wales Conference Final.
"We're right now in practice getting our legs back under us, working on our games ...," Cassidy said. "So you have your hands full. I think that division when you were going into the playoffs, all four teams were in that category, so there was going to be no easy series."
Boston advanced to the second round for a fourth straight season. It lost to the Tampa Bay Lightning in the second round last season.
New York lost the Eastern Conference Final to Tampa Bay last season.
NHL.com staff writers Amalie Benjamin and Brian Compton contributed to this report